Command surfaces, 20. Fixation of surface-assisted homogeneous alignment of nematic liquid crystals by cationic photopolymerization

The surface-assisted alignment of a nematic liquid crystal yields a persistent ordered structure upon cationic photopolymerization of liquid-crystalline epoxides. Linearly polarized light irradiation of a photocrosslinkable polymer containing azobenzene moieties played a crucial role in this procedure since the photoinduced alignment is not deteriorated by heating for polymerization.     

Comparison of two force fields in MD-simulations of α-helical structures in keratins

Molecular dynamic (MD) simulations based on two different force fields, CVFF and CFF91, were carried out in order to check their feasibility for the structural investigation of the wool intermediate filament (IF) monomeric unit. Selecting an ideal α-helix as start conformation, all MD-simulations with CVFF in vaccum show the α-helix to be unstable. Independently of the amino acid sequence of the α-helix, a new helical structure with a larger diameter arises during the MD-simulation, due to a shift of the intrahelical hydrogen bonds. However in simulations with surrounding water the α-helix remains stable throughout the simulations with the CVFF force field. In contrast to this, MD-simulations in vaccume based on the CFF91 force field are able to determine different stabilities for the α-helical start conformation of various IF-segments, that agree well with secondary structure predictions. The simulation results obtained with CFF91 in vacuum can like wise be verified using an explicit water environment. We found that higher partial charges attributed to the atoms of the amide groups that form the intrahelical hydrogen bonds are the reason for the superiority of the CFF91 force field.     

Cyclization reaction of polyacrylonitrile and poly(styrene-co-acrylonitrile) studied by PVT measurements

Pressure, volume, temperature (PVT) measurements reveal that during the intramolecular cyclization reaction of poly(acrylonitrile) (PAN) and poly(styrene-co-acrylonitrile) (SAN) the volume decreases. This volume contraction becomes smaller with increasing styrene content in the random copolymers and should be related to the simultaneous decrease of longer acrylonitrile homo-sequences. The onset temperature of the cyclization reaction is raised with higher styrene contents in the random copolymers. Thus, it can be excluded that the cyclization reaction has a major influence on the discoloration process of SAN samples having relatively small acrylonitrile contents (less than 50 mol-%) during thermal annealing below 300°C.     

NMR chemical shifts in amino acids: effects of environments in the condensed phase

We present calculations of NMR chemical shifts in crystalline phases of some representative amino acids such as glycine, alanine, and alanyl-alanine. We explore the effects of environment on the chemical shifts in selected glycine geometries ranging from the crystalline phase to completely isolated molecules. In the crystalline and dilute molecular limits, the calculated distinct NMR chemical shifts are attributed to intermolecular hydrogen-bonds and dipole electric field effects, respectively.     

"Safe" Coulomb excitation of 30Mg

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient gamma spectrometer MINIBALL. Using 30Mg ions accelerated to an energy of 2.25 MeV/u together with a thin (nat)Ni target, Coulomb excitation of the first excited 2+ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative deexcitation gamma-ray yields the B(E2;0(+)gs-->2(+)1) value of 30Mg was determined to be 241(31)e2 fm4. Our result is lower than values obtained at projectile fragmentation facilities using the intermediate-energy Coulomb excitation method, and confirms the theoretical conjecture that the neutron-rich magnesium isotope 30Mg resides outside the "island of inversion."     

Computing Final Polynomials and Final Syzygies Using Buchberger’s Gröbner Bases Method

Final polynomials and final syzygies provide an explicit representation of polynomial identities promised by Hilbert’s Nullstellensatz. Such representations have been studied independently by Bokowski [2,3,4] and Whiteley [23,24] to derive invariant algebraic proofs for statements in geometry. In the present paper we relate these methods to some recent developments in computational algebraic geometry. As the main new result we give an algorithm based on B. Buchberger’s Gröbner bases method for computing final polynomials and final syzygies over the complex numbers. Degree upper bound for final polynomials are derived from theorems of Lazard and Brownawell, and a topological criterion is proved for the existence of final syzygies. The second part of this paper is expository and discusses applications of our algorithm to real projective geometry, invariant theory and matrix theory.     

Zur Konstruktion eindimensionaler homogener Markoffscher Prozesse

Summary The aim of this paper is the construction of a class of semi-groups of homogeneous onedimensional Markov processes with respect to a given infinitesimal operator. This is done namely by the method of stochastic integration represented in the book of Skorokhod. The Lipschitzian conditions — needed there — are weakened to uniform continuity by a method of approximation of semi-groups. Also to these semi-groups we can construct Markov processes as solutions of stochastic integral equations. The representation is connected with the result of Meyer, Watanabe, Motoo.     

Frobeniusautomorphismen endlicher Gruppen

Ohne ZusammenfassungHerrnGottfried Köthe zum 60. Geburtstag am 25. 12. 1965 gewidmet     

NIR Light-Induced ATRP for Synthesis of Block Copolymers Comprising UV-Absorbing Moieties

NIR exposure at 790 nm activated photopolymerization of monomers comprising UV-absorbing moieties by using [Cu^II/(TPMA)]Br₂ (TPMA=tris(2-pyridylmethyl)amine) in the ppm range and an alkyl bromide as initiator. Some of them comprised structural elements selected either from those showing proton transfer or photocycloaddition upon UV excitation. Polymers obtained comprise living end groups serving as macroinitiator for controlled synthesis of block copolymers with relatively narrow molecular weight distributions. Chromatographic results indicated formation of block copolymers produced by this synthetic approach. Free-radical polymerization of monomers pursued for comparison exhibited the expected broader dispersity of molecular weight compared to photo-ATRP. Polymerization of these monomers by UV photo-ATRP failed on the contrary to NIR photo-ATRP demonstrating the UV-filter function of the monomers. This work conclusively provides a new approach for the polymerization of monomers comprising UV-absorbing moieties through photo-ATRP in the NIR region. This occurred in a simple and efficient pathway. However, studies also showed that not all monomers chosen successfully proceeded in the NIR photo-ATRP protocol.